There are a number of plethysmographs which have been used to measure changes in volume or volumetric flow of portions of the body. In U.S. Pat. No. 3,570,474, apparatus for determining body volume changes is disclosed which utilizes a rigid tubular member for surrounding a portion of a body extremity leaving an annular chamber therearound. The chamber is in communication with a flowmeter which is sensitive to fluid flow into or out of the tube chamber, caused by volume changes in the body extremity enclosed within the tube. Another plethysmograph shown in U.S. Pat. No. 3,381,682 is of the capacitance type. However, the capacitance plethysmograph suffers from sensitivity to environmental conditions and capacitance changes due to variations in skin moisture and dielectric composition of the flesh. The capacitance plethysmograph requires custom-made capacitance transducers which are not easy to calibrate.
A few ultrasonic plethysmographs have also been developed for determining various body changes. One such system is shown in U.S. Pat. No. 3,888,238 for detecting blood flow at various depths under a transducer element. This system detects the flow of blood particles through blood vessels and provides a video output which indicates the flow of blood particles through the vessels. The video output signal is sampled in a display form which indicates the depth, location and the size of underlying blood vessels.
In U.S. Pat. No. 4,095,597, a system is shown for measuring cross-sectional fluctuations of blood vessels through an ultrasound-Doppler method. In this arrangement, a single ultrasound transmitter/receiver is used for projecting ultrasound into the fluid and for receiving the ultrasound reflected by the fluid. Dopler apparatus is provided for producing Dopler signals and for detecting intensity and amplitude fluctuations thereof.
Other ultrasonic apparatus for measuring volumetric flowrates of fluids through pipelines or other conduits are illustrated in U.S. Pat. Nos. 4,024,760, and 4,102,186. However, these devices are not concerned with changes in the volume of the conduit since the cross-sectional area of the conduits is assumed to be constant.